Electron optical system for cathodes of electron beam tubes



Aug. 19, 1952 A. LERBS 2,607,904

ELECTRON OPTICAL SYSTEM FOR CATHODES 0F ELECTRON BEAM TUBES Filed Oct. 11, 1949 5 Sheets-Sheet 1 k [L l Aug. 19, 1952 A. LERBS 2,607,904

ELECTRON OPTICAL SYSTEM FOR CATHODES OF ELECTRON BEAM TUBES Filed Oct. 11, 1949 5 Sheets-Sheet 2 //V YEA/TOE ,4; FRED L ERBS A. LERBS ELECTRON OPTICAL SYSTEM FOR CATHODES OF ELECTRON BEAM TUBES Aug. 19, 1952 3 Sheets-Sheet 5 Filed Oct. 11, 1949 //VVE/V7'0/? 4A FRED 4 5/?55 W ATTORNEY Patented Aug. 19, 1952 UNITED STA-ms 1 I or.

.ELEGTBON :ORTICAL srsrEMF0:a.:-cA'rH-- V onus or ELECTRONVBEAMEUBES' -Alfred; Lerhs, Paris; France, ,to"(3om-'-: I

' pagnie' Generalede Telegraphic sans -Fil; -acorporation-ofalliance" T l I 'Application!)ctoberl1, 1949,;seriaiis6. 412M111] InFrance'October 13, I948- Myinvention relates-'to-electron beam tubes and covers an electron optical system adapted to be applied-to the cathode of such tubes.

It is an object of this invention to provide an improved electron optical system- 'for an electron beam tube.

A further object is to provide a cathode-system capable of producinga large emission current which can be adjusted by contrdllingthe current through the cathode.

These and other objects of the invention will be apparent from the following desz'zri-ption, the appended claims, and the drawings, in which Figs. 1 and 2-show conventional electronbeam tubes o'fthe travelling-wave type;

3 shows-the distribution of the electric field of the cathode structure-shown in Fig.- '2 under certain operating. conditions; and

Figs; 4 through ds'how three-embodiments of my improved electron-optical system.

The drawbacks of the knowncathodes-will be explained with reference-to Figs 1- and 2 which illustrateknown electron beam tubes belonging to-the-class o'ftravelling-wave'tubes with a transverse magnetic field, to-which rev-invention can be advantageously although notexclusively applied. I

In Figs. 1 and "2', a travelling-wave tube; cntained in a straight casing V, comprises in' a known manner a helical-shapedretardation line H with its :input atE- and its output at S, a cathode C, an electrode z' whi'ch is located onposite the retardation line and which is raised to a suitable potential for =producing an electric field between Z and 'I-Land a-collector K raised to a positive potential. The whole arrangement is placed in a. transverse magnetic field- B- set up by a suitablecoil andcore. i r

The cathodes used in these tubes may beeith'er flat-shaped oxide cathodes, orfilament-sh'aped cathodesmade of metal such as tungsten'or tantalum. They areusually raised to a potential that corresponds :to the distribution of same 'in the region between H and Z in which they :are located, so as not to disturb the shapeof. the static linesof-lorce. w

Fig. 1 shows. for example .anoxid'e icathodelocated at the level of the electrode Zand'raised to the same potential as said electrodeza Fig. '2 also shows bywayof example a metal cathode which islocatedinthe space "between-Z- and H andis raised-to an intermediate potential between thesetwo electrodes according to' the potential drop up *to the region occupied by the t e. E. ;.3

items. (01. 315-29) Inthefirst "case '(oXide'cathode) "the emission current cannot" be" adjusted*by-acting on the heating; and furthermore such cathodes "cannot be used if'th e anode'voltag'e-is higher than a certain "maximumflvalue; in the second-case '(meta'l cathodes) the 'eniitting area is small and the emission current is limitedby -the1-space charge. The emission could beincrea'sedby deforming the electric field "around" the cathode in such a' manner "thata greater portion -01? the lines OffOl'CE'flI'B"diIB'Otea"t0WE1JIfdS the cathode fin which" case the increased "field density ad jacen't'the cathode would prevent the formation of a space charge) "for example by placing 'the cathode "as in-Fig; 2 but raising/it to a lower potential than" that'whichcorresponds to its position in'thefield"betweeni-I andZ; Fig. 3 shows the distribution-of the 'electric' held in the case in which the" cathode is" placed as in Fig. 2 but is "raised to' the same potential as the elect-rodeZ. 1 V

However; "this -'arrangerrrentdras-the effect "of influencing" the shapetor-thestati-c clraracteristic of the tube, that shows the-currentin-the helix as a function of the field B, since in this case said current gradually "decreases as 13' increases instead of suddenly 'droppingto zero-whenacertain critical value BC" is "exceededasis' required for the efiicicncy of-the tribe.- 1

These 'diificulties are *obviate'd'by my invention which enables a cathode to be constructed that combines "the advantages -oi t-he fiat oxideoathode internal emission *andgood static char-' acteristic) with-those ofth'e-meta-l cathode( emission, that can be adjusted by-means of the heating and nozlimitation ofthe value of' 'the' anode voltage). m

According to the'invention'fwith "a cathode-of cylindrical shape is associated anelectron optical system comprising a} likewise cylindrical anode that "surrounds the -cathode," and a -merhber adapted to connect the field distributionround the cathode tothe field distribution betwee'n'the retardation line and the electrode which islocated'opposite said line; V

The invention will be more "clearly=understood by referring to 'Figs. l-to '6 which 'showrbywa'y of example n'onlimitatitle embodimentsi thereof.

According to Fig.'i4;in which thesame elements as in Figs. 1 and 2 arexde'signated "by thezsame reference letters, an electron. beamiubelsfor example of the travelling-wave type.;with.a transverse magnetic field andiofzstraightistructm'e, is provided with a cylindricalzl-cathodefi iwhichcis located at the level of thecelectmderzihnd which space the beam is adapted to pass. Byjmeans of this arrangement an intense field is produced ad-- a jacent the cathode, without disturbingthe ho.-.

mogeneous distribution of the field in all directions as in Fig. 3, since it can be seen that within the limits of the sector ab almost all the lines of force e extend radially up to the cathode, so that the density of the field adjacent the cathode surface is high, whereby the magnitude of the" space charge is decreased and the cathode current is increased. On the other hand, the homogeneity of the fieldin almost all directions enables a good static characteristic to be obtained. .It is therefore possible to use a metal cathode'and take advantage of its inherent advantages, to which are furthermore added the advantages which were heretofore possessed'only byoxide cathodes] I But .it is .als'o poss'ible to apply the invention in the case in which. an'oxide cathode is used, which may advantageously be made of cylindrical shape which is' easier to fit when mounting the electrodes than a flat cathode and which moreover is the only shape that gives satisfactory results as regards manufacturing technology in the case of a thorium oxide cathode.

The radius DA of the circular portion is determined by taking into account that the electrons do not touch the anode, such radius being variable and a function. of the magnetic field applied, of the anode voltage and of the distance DH, between the electrodes Z and H, at which the member A is connected to'the point e. The length he should preferably be equal to several times the distance DH. v a

Fig. 5 shows the application of the arrangementsofjig. 4 to 'a travelling-wave tube with a transverse magnetic fieldjand of circular structure. It does not require further explanations since the reference letters' are the same as in Fig. 4.

Fig. 6 shows the application of the principles of the invention to the case in which the cathode is raised to an intermediate potential between Z and H and isarranged as in Fig. 2. In this case, at the levelof the cathode C is placed an auxiliary electrode C raised to the same potential as C, the member A surrounding the cathode as in Fig. 4, and the dimensioning rules being applied with respect to theelectrode C as they were previously with respect to the electrode Z. In other respects, the figure does not require-further explanations, since the reference letters are the same as in Fig.4. v

The invention is not restricted to the examples which have been described and illustrated, but on the contrarymay-be subjected to any modificationithat may be within the scope of the expert, without changing its principle. Thus, for example, it is notnecessary for the cathode to be a linear filament; .It' is'also possible to use a wound filament,-particular1y 'wheniit is desired to have a cathode of' large. area. In that 'case, account must be takenof'thediameter .of the cathode in dimensioningthe system. I

The application of the invention to travellingwave tubes with a transverse magnetic field has only been described by way of a non-limitative example and it is to be understood that the invention can also be applied to travelling-wave tubes without a transverse field, and more generally to any electron beam tube in which it is required to direct in a desired manner a beam emitted by a cylindrical cathode.

I claim:

1. In an electron tube structure, a cathode for emitting electrons, an electrode connected to said ;cathode and; extending substantially radially therefrom, anelectron collector spaced from said cathode, ananode having a cylindrical portion partially surrounding said cathode with the latter positioned at the axis of revolution of said cylindrical portion so that the electric field surrounding said cathode is substantially radial,

said anode having a substantially rectilinear extension extending adjacent said electrode, and means producing a magnetic field extending parallel to the axis'of revolution of said anode cylindrical portion for causing electrons leaving said cathode to flow in a stream between said anode extension and said electrode toward said collector. I

2. An electron tube structure as defined in claim 1 wherein means is provided for setting up an electric field extending between said electrode and said anode extension and substantially transversely of both said electron stream and said magnetic fieldp,

3. An electron tube structure comprising a cathode, a substantially .cylindricalanode substantially surrounding said cathode and-positioned with the latter at its axis 'of revolution for urging electronsradially therefrom in substantially all directions, said anode serving to produce a dense electric field adjacent the'cathode whereby the magnitude ofthe space charge is maintained low and the cathode current is relatively substantial, an electron collector; and an electrode connected'with and extending substantially radially from said cathode and maintained at cathode potential, said anode having an extension located adjacent to but spaced from said electrode for confining the electrons between the extension and said electrode.

4. A tube structure as defined in claim 3 where-- in means areprovided for setting up a transverse magneticfleld for causing the electrons to form a stream flowing from said cathode and longitudinally through-the space between said electrode and said anode extension toward said collector. 1

5. Electron tubestructure comprising a cathode, an electrode extending radially from said cathode, a substantiallycylindrical hollow anode surrounding said cathode and extending into the proximity of said electrode, said cathode being positioned along the axis of revolution of said anode whereby electrons are caused to diverge from said cathode, in substantially'all directions, an electron collector spaced from said cathode, a retardation line located between said cathode and said electron collector adjacent the flight path of electrons moving toward said collector, said electrode also extending between said cathode and said collector adjacent the flight path of the electrons, said electrode being maintained at substantially cathode potential while said retardationlineis maintained at substantially anode potential, means for establishing a magnetic field transversely of the flight path of said electrons,

the potential difference existing between said electrode and said retardation line cooperating with said magnetic field to cause said electrons to flow from the cathode to the collector without collecting either upon the electrode or the said retardation line.

6. Electron tube structure as defined in claim 5 wherein said anode is provided with a substantially straight portion positioned for directing the electrons into the space between said electrode and said retardation line.

7. In combination, a cathode, a cylindrical anode substantially surrounding said cathode with the latter positioned along the anode axis of revolution for producing a substantially uniform radial electric field around said cathode, said anode having an opening therein, and an electrode extending from said cathode through the opening in said anode, said anode being provided with an extension a flattened surface somewhat spaced from and opposed to one surface of said electrode for cooperating with said electrode to guide electrons therebetween.

8. An electron tube structure comprising a cathode, an anode having a cylindrical portion substantially surrounding said cathode with the latter positioned along the axis of revolution of said anode cylindrical portion for producing a substantially uniform radial electric field around said cathode, said anode having an opening therein, an electrode extending from the region of said cathode through the opening in said anode, an electron collector, a retardation line located between said cathode and said electron collector adjacent the flight path of electrons moving from said cathode to said collector, said retardation line having one end adjacent to the opening in said anode, said tube structure having a magnetic field extending substantially transversely of the flight path of electrons and means for providing an additional electric field substantially perpendicular to the axis of said retardation line and coextensive therewith.

9. The apparatus of claim 8, wherein said electric field providing means comprises an extension of said electrode and a source of potential connected between said electrode and said retardation line.

10. The apparatus of claim 8, wherein said electric field providing means comprises an auxiliary electrode and a source of potential connected between said auxiliary electrode and said retardation line, and wherein the electrode which 6 extends from said cathode is maintained at a potential intermediate the potential of said auxiliary electrode and the potential of said retardation line.

11. The apparatus of claim 8, wherein said anode is provided with a flattened extension having a surface spaced from and opposed to one surface of said electrode.

12. The apparatus of claim 11, wherein the portion of said additional electric field adjacent said electrode and said anode extension is substantially parallel to the electric field produced between said electrode and said anode extension by said anode and said cathode.

13. The apparatus of claim 8, wherein said retardation line is in the form of a partial annulus.

14. The apparatus of claim 8, wherein said retardation line extends in a substantially straight line.

ALFRED LERBS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,955,011 Mega-n Apr. 17, 1934 2,414,121 Pierce Jan. 14, 1947 2,475,646 Spencer July 12, 1949 2,511,407 Kleen et a1 June 13, 1950 2,531,972 Doehler Nov. 28, 1959 OTHER REFERENCES The Traveling-wave Tube as Amplifier at Microwaves, by Rudolf Kompfer. Reprinted from the Proceedings of the Institute of Radio Engineers and Waves and Electrons, vol. 35, No. 2, Feb. 1947, pgs. 124-127.

Traveling-Wave Tubes, by J. R. Pierce and Lester M. Field. Reprinted from the proceedings of the Institute of Radio Engineers and Waves and Electrons, vol. 35, [No. 2, Feb. 1947, pgs. 108-111.

Sur les Proprietes des tubes a Champ Magnetique Constant, by J. Brossart and O. Doehler, extract from Annales de Radioelectricite, vol. III No. 14, Oct. 1948, pgs. 328-338.

Sur Laide Que Peuvent Apporter en Television Quelques Recentes Conceptions Concernant Les Tubes Electroniques Pour Ultra-Hautes Frequences, by R. Warnecke and P. Guenard, extract from Annales de Radioelectricite, vol. III No. 14, Oct. 1948, pages 1 through 22. 

